An apparatus and process for forming products from thermoplastic polymeric materials, such products having three-dimensional patterns and surface textures, were disclosed in U.S. Pat. Nos. 4,128,369 and 4,290,248, both of which are hereby incorporated by reference.
In the apparatus and process disclosed in said patents thermoplastic material to be formed was heated above its glass transition temperature before introduction between two opposed travelling flexible mold belts, which were being revolved in opposed relationship. The flexible mold belts each included a thin, flexible sheet-metal layer of relative high thermal conductivity. At least one sheet-metal layer had a rubber mold bonded thereon for providing a flexible three-dimensional patterned mold formed on the front face of the mold belt. When two such mold belts were revolved in opposed relationship along their respective upper and lower oval paths with their mold surfaces in opposed face-to-face relationship both moving at the same speed, the two mold belts provided a travelling mold channel between their mold surfaces.
At the entrance to the travelling mold channel opposed nip rolls pressed the upper and lower revolving mold belts against entering hot thermoplastic material in a progressive localized rolling, squeezing action in the nip region. Thereafter, a series of backup rollers arranged along the travelling mold channel held the moving mold belts against the impressed hot plastic material located between them for maintaining the impression while being cooled.
Mold belts were shown in those patents as each including a wide, thin, endless, flexible sheet-metal layer, at least two-feet wide. At least one of two opposed mold belts had a wide, flexible mold formed of a heat-resistant material, such as rubber, bonded to the thin metal layer. In practice, the mold belts in the prior disclosed apparatus each included an extremely thin sheet-steel layer, being typically 0.025 to 0.075 of an inch thick. The thin steel layer was the inside surface layer of the flexible mold belt and had a silicone rubber mold bonded onto the outside of the thin steel layer for providing a three-dimensional patterned mold surface. The steel layer, being on the inside of the mold belt, revolved in contact with large diameter metal pulleys, i.e., a steel layer running in direct contact with a metal-surface pulley. Such an extremely thin steel layer located on the inside surface of a revolving mold belt is susceptible to dents, crimped edges and rust. Also, such a thin steel layer on the inside surface of a flexible revolving mold belt presents difficulties in maintenance of alignment in travelling over metal pulleys.
In the prior disclosed apparatus, many small-diameter backup rollers arranged along the travelling mold channel were rolling in contact with the thin steel layer of each of the opposed mold belts. These small-diameter rollers caused the travelling, opposed, upper and lower, revolving flexible mold belts to experience considerable fluctuations in contact pressures against the thermoplastic material located between them as they successively passed over roller-gap-roller-gap-roller, etc.
In practice, the prior disclosed apparatus utilized cooling liquid (water which may contain corrosion-inhibitors) applied to the back (inside surface) of the thin, thermally-conductive sheet-steel layer of both mold belts. The numerous small-diameter backup rollers were arranged to permit such liquid coolant to flow along the sheet-steel layer of each mold belt for extracting heat through this metal layer of the mold belt. After the thermoplastic material product had become sufficiently cooled to retain its impressed three-dimensional patterns, the revolving mold belts separated from the continuously formed 3-D product as the product progressed out from an exit end of the apparatus.
The prior disclosed apparatus utilized hydraulic actuators to tension the mold belts, provide a compressive force to maintain contact of the two opposed, travelling mold belts, and to lift the upper mold assembly off of the lower mold assembly to facilitate maintenance and the changing of mold belts.
As is well known in the art, hydraulic actuators require the use of a motor, pump, various hoses and valves, and actuator cylinders. The hydraulic system must be maintained in a leak-free condition in order to function properly and prevent contamination of the molded product. Hydraulic systems constantly consume electrical energy when the apparatus is operative. That is, the hydraulic motor and pump must be running often in order to provide pressure to maintain and change positions of the hydraulic actuators during operation of such continuous 3-D forming apparatus. The motor and hydraulic pump are inherently noisy and commonly are located in close proximity to the apparatus, which makes the working environment of such prior apparatus uncomfortable.